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Arabidopsis thaliana exhibits a developmentally regulated disease-resistance response known as age-related resistance (ARR), a process that requires intercellular accumulation of salicylic acid (SA), which is thought to act as an antimicrobial agent. ARR is characterized by enhanced resistance to some pathogens at the late adult-vegetative and reproductive stages. While the transition to flowering does not cause the onset of ARR, both processes involve the MADS-domain transcription factor SHORT VEGETATIVE PHASE (SVP). In this study, ARR-defective svp mutants were found to accumulate reduced levels of intercellular SA compared with wild type in response to Pseudomonas syringae pv. tomato. Double mutant and overexpression analyses suggest that SVP and SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CO 1) act antagonistically, such that SVP is required for ARR to alleviate the negative effects of SOC1 on SA accumulation. In vitro, SA exhibited antibacterial and antibiofilm activity at concentrations similar to those measured in the intercellular space during ARR. In vivo, P. syringae pv. tomato formed biofilm-like aggregates in young susceptible plants, while this was drastically reduced in mature ARR-competent plants, which accumulate intercellular SA. Collectively, these results reveal a novel role for the floral regulators SVP and SOC1 in disease resistance and provide evidence that SA acts directly on pathogens as an antimicrobial agent. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Systemic acquired resistance (SAR) is a plant defense response that provides long-lasting, broad-spectrum pathogen resistance to uninfected systemic leaves following an initial localized infection. In Arabidopsis (Arabidopsis thaliana), local infection with virulent or avirulent strains of Pseudomonas syringae pv tomato generates long-distance SAR signals that travel from locally infected to distant leaves through the phloem to establish SAR In this study, a proteomics approach was used to identify proteins that accumulate in phloem exudates in response to the induction of SAR To accomplish this, phloem exudates collected from mock-inoculated or SAR-induced leaves of wild-type Columbia-0 plants were subjected to label-free quantitative liquid chromatography-tandem mass spectrometry proteomics. Comparing mock- and SAR-induced phloem exudate proteomes, 16 proteins were enriched in phloem exudates collected from SAR-induced plants, while 46 proteins were suppressed. SAR-related proteins THIOREDOXIN h3, ACYL-COENZYME A-BINDING PROTEIN6, and PATHOGENESIS-RELATED1 were enriched in phloem exudates of SAR-induced plants, demonstrating the strength of this approach and suggesting a role for these proteins in the phloem during SAR To identify novel components of SAR, transfer DNA mutants of differentially abundant phloem proteins were assayed for SAR competence. This analysis identified a number of new proteins (m-type thioredoxins, major latex protein-like protein, ULTRAVIOLET-B RESISTANCE8 photoreceptor) that contribute to the SAR response. The Arabidopsis SAR phloem proteome is a valuable resource for understanding SAR long-distance signaling and the dynamic nature of the phloem during plant-pathogen interactions., (© 2016 American Society of Plant Biologists. All Rights Reserved.)

Systemic acquired resistance (SAR) is a plant defense response in which an initial localized infection affords enhanced pathogen resistance to distant, uninfected leaves. SAR requires efficient long-distance signaling between the infected leaf, where SAR signals are generated, and the distant uninfected leaves that receive them. A growing body of evidence indicates that the lipid transfer protein DIR1 (Defective in Induced Resistance) is an important mediator of long-distance SAR signaling. In a recent publication, we investigated if cell-to-cell movement through plasmodesmata is required for long-distance movement of DIR1 during SAR. We determined that overexpression of Plasmodesmata-Located Proteins (PDLP1 and 5) negatively impacted long-distance DIR1 movement and SAR competence, suggesting that movement through plasmodesmata contributes to long-distance signal movement during SAR.

The phytohormone salicylic acid (SA) plays an important role in several disease resistance responses. During the Age-Related Resistance (ARR) response that occurs in mature Arabidopsis responding to Pseudomonas syringae pv tomato (Pst), SA accumulates in the intercellular space where it may act as an antimicrobial agent. Recently we measured intracellular and intercellular SA levels in young, ARR-incompetent plants responding to virulent and avirulent strains of Pst to determine if intercellular SA accumulation is a component of additional defense responses to Pst. In young plants virulent Pst suppressed both intra- and intercellular SA accumulation in a coronatine-dependent manner. In contrast, high levels of intra- and intercellular SA accumulated in response to avirulent Pst. Our results support the idea that SA accumulation in the intercellular space is an important component of multiple defense responses. Future research will include understanding how mature plants counteract the effects of coronatine during the ARR response.